Researchers
have used DNA microarrays to compare 36 strains of the bacterium Staphylococcus
aureus, a human pathogen that can cause life-threatening infections,
particularly among hospital patients. The content of the 36 genomes varied
widely, and this variation may be relevant to efforts to combat the pathogen,
the researchers say.

The DNA microarray used by James M. Musser, of the US National Institute
of Allergy and Infectious Diseases, and colleagues contained more than
90 percent of the genome of the strain S. aureus COL. The researchers
identified 18 large chromosomal regions that differed significantly among
the strains. Ten of these contained genes linked to the bacterium's virulence
and its ability to resist antibiotics.

The researchers screened eleven strains that are resistant to methicillin,
the antibiotic commonly used to treat S. aureus infections in humans.
Methicillin-resistant strains appear to have acquired genes involved in
antibiotic resistance directly from other bacterial strains, a biological
phenomenon known as horizontal gene transfer.

"It is clear that horizontal gene transfer has played a fundamental
role in the evolution of pathogenic S. aureus," the researchers
write in Proceedings of the National Academy of Sciences.

Strains of S. aureus that cause toxic shock syndrome were also
analyzed. Toxic shock syndrome is a rare blood infection most commonly
associated with women who use highly absorbent tampons (although it can
affect men and children). Nine of the strains in this study were cultured
from women with the illness. The reasons for an epidemic of toxic shock
syndrome in the U.S. in the 1970s are unknown, but the microarray analysis
suggests that a new virulent strain of S. aureus was not the cause.

"The epidemic of toxic shock syndrome that occurred in the 1970s
was caused by a change in the host environment, rather than rapid geographic
dissemination of a new hypervirulent strain," the researchers write.

The 36 strains in the study were among the most abundant identified during
a survey of 2,077 S. aureus isolates from around the world. Also
in the study were strains that cause infectious disease in sheep and cattle.

The research team included Steven R. Gill, of The Institute for Genomic
Research (TIGR) in Rockville, Maryland. A pre-publication view of the
Staphylococcus aureus COL genome sequence is available at TIGR's
Web site.